The present invention relates to a method for tire pressure warning in a motor vehicle.
In principle it is known, in order to save having other systems which are complex in terms of sensors, to carry out a warning of incorrect, i.e. too low or even too high, tire air pressure during travel by evaluating the dynamic tire rolling circumferences, i.e. from wheel speed sensor signals. This is advantageous particularly if appropriate sensors are already available in the vehicle, for example as ABS speed detectors. The dynamic rolling circumference of a wheel changes as a function of the tire air pressure and thus leads to different wheel rotation speeds during slip-free, straight-ahead travel. An air pressure drop from 2.4 bar to 1.9 bar, for example, causes a wheel speed which is faster by about 0.3% at the wheel with too low air pressure within a mean speed range between 60 km/h and 100 km/h despite the vehicle speed remaining the same. An air pressure loss can therefore be derived from the wheel speed differences. Using this method, a relative air pressure loss from one wheel to another can be established.
In a warning method according to U.S. Pat. No. 3,707,701, a signal is produced for each full revolution of a wheel, and the phase relation between the signals from different wheels is observed, a warning report taking place in the case of the period length being too short.
A tire pressure warning method for airplane tires is known from U.S. Pat. No. 4,355,298, in which the wheel speeds of the two tires are recorded and compared with one another, the resulting difference, in turn, being compared with a reference value. The momentary transverse speed is thus intended to be taken into consideration in order to prevent a speed difference, which is still relatively small at low speeds and is based not on a pressure loss but on a different interfering parameter, e.g. unequal static tire circumference, leading to the pressure warning responding incorrectly at relatively high speeds.
A difficulty in this type of indirect tire pressure determination results from the fact that wheel speed differences can also occur in the case of correct tire air pressure, e.g. due to braking or driving slip, cornering, differing axle load distribution, loading, use of tires of different design, make and/or running performance, as well as temperature differences and differing "swelling" of the tires caused by centrifugal force at a relatively high speed. These effects must be taken into consideration as far as possible for tire pressure warning by means of wheel speed measurement.
A tire pressure control method disclosed in the German Patent Document DE 4,102,769 A1 provides for dynamic wheel circumference differences, which are caused by static tire circumference differences, to be determined by means of a separate calibration journey with straight ahead travel at a correctly set air pressure and to be stored in a computer. Cornering is taken into consideration in this method by means of a special cornering model which results in two wheel frequency equations. Wheel slip or loading-related speed differences are compensated by introducing a parameter which can be eliminated in this air pressure control model in such a way that the evaluating computer treats the input parameters in a single equation as a control function, an air pressure loss being assumed if this control function assumes values above a predetermined threshold value for a relatively long time.
In U.S. Pat. No. 3,691,524, the influence of cornering is counteracted by the fact that a warning report takes place only if a speed difference occurs either only between the front wheels or only between the rear wheels. At the same time, a warning report is suppressed if the speed difference detected exceeds a specific value since this is interpreted as spinning of the relevant wheel.
The variant of a tire pressure warning method known from U.S. Pat. No. 3,613,075 provides for the warning system to be suppressed at too low a speed since in this case it is assumed that spinning of the wheels occurs above all within this low speed range.
Tire pressure warning methods are known from European Patent Documents EP 0,291,217 B1, EP 0,441,599 A2 and EP 0,441,600 A2, which methods, while individually differing in the recording of the wheel speeds, all produce a warning signal on the basis of the same mathematical relations. One of these relations includes, inter alia, essentially the sum of the two differences between the wheel speeds of the wheels on the same axle. This procedure allows the sensor system and control of an antilock brake system (ABS) also to be used at the same time for the tire pressure warning and it automatically takes cornering into account. It is additionally known from these publications to suppress the tire pressure warning during braking and to carry out a calibration during drive-free, straight-ahead travel at more than 20 km/h. Based on the evaluation arithmetic selected in these documents, however, a virtually simultaneous tire pressure drop, for example, can only be recognized at diagonally opposite wheels, but not at wheels on the same axle or on the same side.
An object of the present invention is to provide a reliable tire pressure warning method which also takes into account the different types of influences on the tire pressure in the best possible manner with a given outlay.
This and other objects are achieved by the present invention which provides a method for providing a tire pressure warning in a motor vehicle, comprising the steps of: a) performing a wheel speed balance in which wheel speed scaling factors for each wheel, monitored in terms of tire pressure, are formed by measuring wheel speeds during travel of the motor vehicle without braking above a minimum speed with a degree of cornering below a predetermined limit value for cornering and a driving torque below a predetermined limit value for driving torque; b) combining the wheels, monitored in terms of tire pressure, in pairs and calculating a difference of the associated wheel speed scaling factors for each pair of wheels; c) determining a greatest difference of the wheel speed scaling factors between the pairs of wheels; d) checking whether the greatest difference determined is greater than a predetermined threshold value, and if so: d.1) repeating steps a) to d) if a predetermined number of cycles has not yet been reached, and otherwise, d.2) output a warning signal to display an excessive tire pressure deviation.
The present invention provides the basis for taking the wheel speed differences into account which are not caused by a significant tire pressure deviation. In this case, the wheel speed balance which is carried out first eliminates the deviations otherwise occurring due to braking and driving slip and cornering. In such a wheel speed balance, the wheel speeds are measured during travel as soon as suitable conditions are recognized during travel which allow the tire circumference to be recorded broadly independently of slip and bends. For this purpose, suitable limit values are prescribed and a balance is initiated when there is a sufficient amount of straight ahead travel with little driving torque above a particular minimum speed and without braking. Such a wheel speed balancing method, such as is used as a basis for the present tire pressure warning, is described in greater detail later. In this wheel speed balancing method, wheel speed scaling factors are formed, by means of which the measured wheel speeds, which may possibly differ from one another even in the case of rolling straight ahead without driving torque, are balanced with one another, i.e. corrected wheel speeds which have been approximated to one another are calculated, which can be fed as input parameters to a subsequent driving or wheel slip control system, such as a permanent four-wheel drive, ABS or-driving slip control (DSC), and also to a control for the present tire pressure warning method. This automatic, sensitive speed balance of the wheels, in which a precision of at most 0.1% deviation of all four corrected wheel speeds with one another is achieved for slip-free straight-ahead rolling, forms the basis of the present tire pressure warning method which is capable of reliably recognizing a pressure loss above 0.5 bar and, at the latest, at 0.8 bar depending on the outlay and sensor system. This tire pressure warning method can be used particularly advantageously in vehicles which already have the appropriate sensor system, e.g. those with a permanent four-wheel drive and/or ABS and/or DSC.
By combining the wheels in pairs and calculating the difference between the associated scaling factors and carrying out the following evaluation of the resultant differences, a tire pressure loss or even excess tire pressure can be recognized reliably in each case in one of the two wheels. The steps of the method are expediently carried out with a predetermined number of cycles which can be, in particular, considerably greater than one so that repetition of the steps of the method takes place in order to check whether a detected deviation is also reproduced in a subsequent determination or is based merely on a single incorrect measurement or a singularity in the driving state, e.g. driving over a curb.
Certain embodiments of the invention provide for the wheel speed balance to be carried out in each case in pairs with wheels on the same axle. This eliminates influences on the tire rolling circumference as a result of differing loading and axle load distribution and as a result of different makes, types and profile depths of the tires, assuming the customary procedure of mounting tires of the same make and the same type and with the same running performance at both ends of one axle. As a result, at the same time differing "swelling" of tires of different makes and types at a relatively high speed are also eliminated. As long as there is no mixing of the tires on one axle, the two tires behave in a comparable manner in terms of their dynamics such that forming the difference eliminates their respective effect on the dynamic tire rolling circumference.
Certain embodiments of the invention have an advantageous effect on the rapidity of a tire pressure warning in that the number of cycles is preselected as a function of the maximum difference, in particular in such a way that, in the case of a relatively large difference, fewer repetition cycles are driven. This is because a greater maximum difference between two scaling factors already allows a tire pressure abnormality to be recognized with greater certainty than a comparatively small difference which, in terms of magnitude, almost fits into the range of normal scaling factor differences with a correct air pressure.
Certain advantageous embodiments of the method according to the present invention provide for the tire which has the pressure deviation to be determined additionally after the output of a tire pressure warning signal so that this can be indicated to the driver, e.g. by means of an appropriate information display unit.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.